The present invention relates to air-cooled components, such as airfoil components of gas turbine engines. More particularly, this invention is directed to a method for sizing and forming the cooling holes of such a component protected by a diffusion environmental coating, in which the build up of the coating within the cooling holes can be predicted and accommodated to ensure proper cooling air flow through the cooling holes.
Airfoils of turbine blades and vanes of gas turbine engines often require a complex cooling scheme in which cooling air flows through the airfoil and is then discharged through carefully configured cooling holes. The performance of a turbine airfoil is directly related to the ability to provide uniform cooling of its external surfaces. Consequently, the control of cooling hole size and shape is critical in many turbine airfoil designs because the size and shape of the opening determine the amount of flow exiting a given hole, its distribution across the surface of the component, and the overall flow distribution within the cooling circuit that contains the hole. Other factors, such as backflow margin (the pressure delta between cooling air exiting the cooling holes and combustion gas impinging on the airfoil) are also affected by variations in hole size. As a result, diametrical tolerances for airfoil cooling holes are typically on the order of about xc2x10.002 inch (about xc2x10.0508 mm) or less. To achieve these tolerances, precision drilling techniques such as laser beam machining, electrical discharge machining (EDM) and electrostream (ES) drilling are typically used to form the holes. Once formed, subsequent airfoil manufacturing operations must be carefully performed so that the dimensions of the holes are not significantly altered.
Due to the severity of the operating environment of turbine blades and vanes, environmentally protective coatings are typically applied to the airfoils of these components when manufactured, and often reapplied during their repair. Diffusion aluminides coatings and MCrAlY coatings overcoated with a diffusion aluminide coating are widely used in the gas turbine engine industry as environmental coatings for airfoils. Diffusion aluminide coatings are produced by aluminizing the airfoil surfaces by such known methods as pack cementation, vapor (gas) phase (above-pack) aluminiding (VPA), chemical vapor deposition and slurry coating techniques. Each of these processes generally entails reacting the surfaces of the airfoil with an aluminum-containing composition to form two distinct zones, an outermost of which is an additive layer that contains the environmentally-resistant intermetallic phase MAl, where M is iron, nickel or cobalt, depending on the substrate material (e.g., mainly xcex2(NiAl) if the substrate is Ni-base). The chemistry of the additive layer can be altered with such as elements as chromium, silicon, platinum, palladium, rhodium, hafnium, yttrium and zirconium in order to modify the environmental properties of the coating. Beneath the additive layer is a diffusion zone comprising various intermetallic and metastable phases that form during the coating reaction as a result of diffusional gradients and changes in elemental solubility in the local region of the substrate.
As is apparent from the concerns discussed above regarding cooling hole dimensions, diffusion aluminide coatings must not interfere with the airflow requirements of an airfoil, and more specifically, the cooling air flow that exits each cooling hole at the airfoil surface. As a result, the cooling holes of an airfoil must be formed in an oversize condition in anticipation of the thickness of the additive layer of the aluminide coating, or care must be taken to avoid aluminizing of the cooling holes. Likewise, aluminizing of airfoils returned for repair must also be performed with care to avoid or minimize aluminizing of the cooling holes and internal surfaces of the airfoils.
The procedure for establishing the proper pre-aluminized diameter of a cooling hole for production airfoils has been to machine cooling holes of different diameters in otherwise production airfoils, perform multiple coating operations under different coating conditions on the airfoils, and then test the airfoils to verify their airflow properties and determine what diameters and coating conditions yield the cooling hole diameters that achieve the required airflow properties. The latter entails sectioning the cooling holes to measure the hole diameters and examine the adequacy of the aluminide coatings. While reliable and accurate, this process requires the use of production hardware and long cycle times, and relies on a hit-or-miss method that is often laborious and tedious.
In view of the above, it would be desirable if an improved process were available for sizing and forming the cooling holes of gas turbine engine components protected by a diffusion coating.
The present invention provides a method for accurately sizing and forming the cooling holes of an air-cooled gas turbine engine component, such as a turbine blade or vane. The invention is particular directed to air-cooled components in which one or more cooling holes having specified diameters must be formed to achieve preestablished cooling airflow conditions, and on which a diffusion coating must be deposited to protect the component from its harsh operating environment. While the material of a component and the parameters of a diffusion aluminizing process have been understood to influence the thickness of the additive layer of a diffusion aluminide coating, and therefore affect the final diameter of a cooling hole in the component, the present invention is based on the determination that different drilling techniques used to form cooling holes also affect the final diameter of a cooling hole.
The method generally entails forming a hole in a surface region of a substrate, and then measuring the thickness of any recast surface region surrounding the hole and created during forming of the hole as a result of a portion of the surface region having melted and then resolidified. According to the invention, an inverse relationship has been determined to exist between the thickness of the recast surface region formed during drilling of a cooling hole and the growth rate of the additive layer under given deposition parameters. As such, by determining the thickness of the recast surface region on the substrate and the growth rate of a diffusion coating deposited on the substrate under a certain set of deposition parameters, the thickness of the additive layer of the diffusion coating deposited on the component can be predicted. Knowing the final diameter required of the cooling hole, an appropriately-oversized hole can be formed in the component so that, after depositing the diffusion coating on the component, the additive layer grows sufficiently within the hole to yield a cooling hole approximately having the required final diameter.
The process of this invention is useful for newly manufactured airfoils and presumably for repaired airfoils, and avoids the deposition of an additive layer that is excessively thick or thin, which would result in insufficient or excessive (respectively) airflow through the cooling holes of the airfoil, thus adversely impacting the airflow and flow distribution through the airfoil. As a result, a notable advantage of the present invention is that it succeeds in maintaining required cooling hole dimensions for an air-cooled component protected by a diffusion aluminide coating, while significantly reducing the labor and time required to arrive at an optimal pre-aluminized cooling hole dimension.
Other objects and advantages of this invention will be better appreciated from the following detailed description.